Process of and apparatus for printing or copying by electrostatic photography



p 13, 1962 R. G. PRITCHARD 3,054,897

PROCESS OF AND APPARATUS FOR PRINTING 0R COPYING BY ELECTROSTATIC PHOTOGRAPHY Filed Aug. 16, 1957 2 Sheets-Sheet 1 ill Inventor RAYMOND GE RGE PRITCHRRD By MM, Lav

Attorneys p 1962 R. c. PRITCHARD 3,054,897

PROCESS OF AND APPARATUS FOR PRINTING OR I COPYING BY ELECTROSTATIC PHOTOGRAPHY Filed Aug. 16, 1957 2 Sheets-Sheet 2 Inventor RHYNO ND GEORGE PR ITCH HRD 7 0 r By 444 6014 14% Attorney;

tates Unite The present invention relates to the process of and apparatus for printing or copying by electrostatic photography, and is particularly although not exclusively suitable for processes where electrostatic charging, exposure and printing by contact or projection, and developing ocour on paper in continuous yardage.

Processes have been proposed employing a photosensitive element comprising a finely divided photoconductor dispersed in an insulating binder and deposited as a layer on a suitable carrier material and it has recently been proposed to produce a photo-sensitive element consisting of paper sheet material coated with a powdered photoconductor in the form of zinc oxide dispersed in a resin binder. A uniform electrostatic charge is imparted to the coated surface in the dark (or under suitable safe-light conditions) by means of a corona discharge from for example an array of fine tungsten Wires connected to the negative terminal of a suitable (e.g., 47 kilovolt D.C.) source whilst the paper is supported on a grounded plate, coated surface remote from the plate, the plate being connected to the positive side of the source. Such papers are now available which are electrically conductive when for example a certain amount of moisture is retained in the paper base. The sheet, now sensitive to light, and the original to be copied are exposed and the electrostatic charge is reduced or eliminated in the exposed areas and retained in the masked areas to form a latent electrostatic charge image on the surface of the paper. The latent image is developed by applying a toner powder, e.g., a pigmented resin carrying a positive electrostatic charge which is attracted to and held by the negatively charged image areas, the powder being subsequently fused to the paper surface to produce a durable light-fast image.

The method of charging that has been proposed is by means of a negative electrostatic charge on the coating side of the paper employing a voltage doubler circuit, but the means employed have the disadvantages of being dangerous to the operator and of not producing the definition that is to be desired, and there is a tendency for the paper to adhere to the ground plate so that the treatment of continuous yardage is made diflicult.

The present invention aims to provide a process whereby sheet or web material sensitized for electrostatically charging as aforesaid can be passed in continuous form through the charging, exposing and developing apparatus.

It has been proposed to dispense with the ground plate and use a double charging unit consisting of opposite corona producing grids, e.g., of a number of tungsten wires of 0.001 inch in diameter, the opposing grids being connected to opposite sources of high-voltage D.C. polarity.

The present invention proposes to employ a charging means whereby positive E.M.F., is employed to one side of the material and negative is applied to the other side of the material by means of charging devices supplied by radio frequency or electron coupled oscillators and preferably the negative high-voltage output is in excess of the value obtained from the positive source. A positive of 6-kv. to 9-kv. DC. potential has been found to be suitable, employing an electron coupled oscillator generating a frequency of from 16 to 30 kcs., e.g., l8 kcs. A range of 17-20 kcs. is preferred.

atent O The current flowing through the wires of the charging devices is dependent upon the material of the wires and their diameter and the distance the wires are positioned from the carrier material. Although we have obtained useful results with wires of stainless steel and platimumclad molybdenum of from 0.001" to 0.003" in diameter we find that tungsten wires of a diameter within this range and especially of 0.002" have given the best results when the positively charged wires are .875 to 1.25 inch and the negatively charged wires are .875 to 1.25 inch from the back and coated sides respectively of the carrier material. This can be more clearly understood by reference to the accompanying drawings where-in FIGURES 1 and 2 are schematic wiring diagrams of the oscillator-rectifier means employed in the present invention for separately generating positive and negative potential, and FIGURE 3 is a cut-away view, partially in section, of the corona charging devices which are electrically interconnected with the oscillator-rectifier means illustrated in FIGURES 1 and 2.

Each unit of the dual corona charging unit may comprise a number of tungsten wires 20 (FIG. 3) e.g., three, of 0.002 inch diameter to which are attached spring connectors (21) mounted on insulated supports shielded by a brass shield or reflector spaced say 0.5 inch from the plane of the wires and between each wire. The two shields or reflectors may be positioned so that the wires are parallel with each other, held in place by two earthed shields which maintain the critical field at the wires and thus enable a uniform discharge to be produced. A nearly complete closure shield may thus be provided with slots 22 in opposed side walls for the continuous passage of the material. The spring connectors may be adjusted by screw-nut means 23 to regulate the spacing of the wires from the material.

The electron coupled oscillator and the power pack therefor (see FIG. 1) for generating the positive output to the one charging unit includes a power pack A of conventional design and the electron coupled oscillator B is designed to generate a :frequency of 18 kcs. The power pack comprises a transformer d the primary 2 of which is tapped to provide for a range of supply voltages, the transformer possessing two secondary windings, 3, 4, one end of the main winding 3 being connected to the negative terminal of a metal rectifier 5 and the other end of the winding being earthed with one end of the auxiliary winding 4, the other end of which is connected to the heater filament 6 of a beam tetrode V1 of the generator.

The generator is an electron coupled oscillator consisting of a tuned circuit incorporating the basic inductance or primary L1 for the output and condenser C6 and a feed back winding L3 for creating oscillation. L2 forms an overwind giving increased voltage to the anode of rectifier valve V2 and the rectified current is smoothed by the resistance condenser arrangement R3, C4 and C 5. This smoothing circuit construction forms a characteristic filter network having a storage energy capacity which is calculated as being of the order of about .072 joule. The heater filament 7 of valve V2 is supplied from L4 which is close coupled to L1 and the desired positive output is obtained from the connection at the junction of R3 and C5. In the experimental circuit the following values were used; C1, 0.001 mf. (microfarad), C2, C3, 0.05 mf., C4, C5, 0.001 mi. 12 kv., C6, 500/ pf., pico- 'farad), R47K--%w., R2, 33K1w.-2w., R3, 100K %w. A 6.v.6 or E.L.41 valve (British identification) will serve as V1 and an E.Y.51 or U37 for V2.

Regulation for voltage can be carried out by adjusting the high tension input and/ or by adjusting the trimmer condenser C6.

The power pack which operates the generator (see FIG. 2) for negative output may be of a conventional design including valve rectifier (V1) in contrast to the positive unit which employs a metal rectifier. This is done in order that the negative high voltage output can be conveniently made in excess of the value obtained from the positive source.

The generator is an electron coupled oscillator comprising valve V2 and tuned circuit C6 and L1 to create RF. output. In order to obtain the increased voltage L2 is an overwind which feeds step up voltage to the filament of V3 which is heated by the winding L3 connected to L2. The desired E.H.T. negative potential is derived from the anode of V3 and is smoothed by RS, C4 and C5.

The combination of the two systems forms a dual corona unit, the action of which injects into the back of the paper a positive charge while a negative charge is imposed on the coating of the paper and in doing so it is possible to neutralise the negative charge of the paper.

One way of carrying out the process, employing the inethod of charging according to the invention is as folows:

An ordinary paper base material is used which is coated on one face with a photo-conductive layer composed of Zinc oxide in a silicone resin binder, a form of electrosensitive medium that has already been developed by the RCA. of America. By way of example, the layer is of the following composition.

192 gms. zinc. oxide 179 gms. toluene 110 gms. silicone resin (MS 2701) The paper may be supplied in the roll and drawn through apparatus comprising the charging devices, an exposure station, a developing and a fixing station.

The electrophotographic paper is first made sensitive to light as aforesaid applying to the back of the paper a positive electric charge and to the sensitive side of the paper a negative charge. This procedure is carried out in the dark or under suitable safe-light conditions.

Following the charging operation, the carrier material is moved on to the exposure station, where by projection, contact or otherwise, the sensitive layer is exposed and the latent image is formed. The exposure time is dependent on several factors including the grade of paper used and the light source employed. For example, white oxide coated paper we have experimented with has a maximum spectral response of 3650 A. However certain organic dyes, such as rose bengal, can impart a higher spectral response to the white zinc-oxide-resin coating extending into the visible range. Further the poor electrical properties in the dyes may increase the dark decay rate of the photosensitive coating. Extensive research has shown that the method of charging herein described is effective with various grades of paper employing a zinc-oxide silicone-resin coating.

In developing the image it is advantageous to create the best differential from image to background for any given exposure, and the initial charge on the paper may be sufficiently high, as, by appropriately choosing the charging time, to obtain this result.

In order to obtain the desired result, a condition is sought in which the toner (developer) is not attracted to the exposed partially discharged areas. This may be achieved by employing a developer powder which includes finely divided ferromagnetic material. The toner powder is then attracted through the medium of a brush of magnetic particles on to the charge image and the resin powder can be subsequently melted so that it fuses to the paper. The toner powder may be carbon pigmented.

A toner mix having the following composition has given good results:

50 gms. piccolastic resin 4358 A. 3 gms. rajah carbon 3 gms. spirit nigrosine 2 gms. iosol black i- Fuse all the chemicals at 200 C. Chip out when cool and grind roughly to about Mill for 24 hours in a ball mill. Grind to pass through a 200 mesh sieve.

According to one developing method which we may employ, the surface bearing the latent image is presented towards a distributed supply of a mixture of ferromagnetic particles and toner particles and a number of magnetised members are arranged with poles adjacent to the back of the paper material, the ferromagnetic particles are magnetised by the magnetised members and attract the toner particles with them to the said surface, and by relative movement between the magnetised members on the one hand the carrier material on the other the toner particles are brushed over the said surface, whereupon the magnetised members are rendered inoperative or deenergised, the ferromagnetic and unwanted toner particles fall away from the surface and a deposit of toner particles is left on the latent image and is caused, as by a fusion process, firmly to adhere to the image. Developing is carried out in darkness or under suitable safe-light conditions. An arrangement of electro-magnets is preferred. A series of ferromagnetic, e.g., soft iron, plates may be energised by means of a common coil within the internal field of which the plates are arranged. These plates which may be at right angles to the plane of the paper material and may have booked or curved ends adjacent the back of the paper material, may be arranged over a band-like area of the back of the paper material attached to and depending from a supporting rod or the like of non-magnetic material such as brass, the coil being in elongated form and surrounding the plates. The rod or the like is reciprocable longitudinally and the paper passed under the plates in a direction at right angles to the rod. Thus as the paper material is advanced under the magnetised plates the latter are given a reciprocating movement at right angles to the direction of movement of the said material, the magnetic particles are attracted with the toner particles upwards onto the under surface of the paper material bearing the latent image and by a magnetic brushing action the toner particles are distributed over the said surface, the whole surface being effectively covered if the magnets operate as a series across the full width of the element. When the surface has been covered the current :is cut off from the coil and the magnetic particles and excess of toner powder fall away from the surface.

The rod may be supported in end bearings so that it can be turned if desired to swing the plates away from their normal operative position.

The developing apparatus may co-operate with a tray in which a mixture of powder iron and toner particles is held and the series of magnets or magnetisable members are reciprocated above the tray, the sensitive paper material passing under the magnets or magnetisable members and over the tray containing the magnetic and toner particles.

The paper material may be displaced in a horizontal plane under the lower ends of the magnetised plates and over the tray of powder material with the surface bearing the latent image facing the powder and whilst the said material is being so displaced the plates are reciprocated over the other surface of the material at right angles to the movement thereof preferably in small spaced relation thereto, so as to effect a satisfactory distribution of the powder over the latent image bearing surface.

Provision may be made to keep the powder satisfactorily mixed in the tray. A mechanical agitator may be used for this purpose and the magnetisable members may be on a common carrier, e. g., a brass rod as aforesaid reciprocated by mechanical means. Mechanical means may be provided for feeding the sensitive paper material through the charging, exposure and developing stations, i.e., in appropriate step-by-step movement.

The developer may be one which adheres only to the discharged portion of the carrier surface.

The developer may be such as will retain a lithographic ink so that a lithographic plate may be produced. Thus the developer powder may be of a hydrophobic nature with the image on a hydrophilic background.

What I claim is:

1. Apparatus for electrostatically charging a photosensitive layer on a carrier material comprising, opposed spaced corona charging devices adapted to permit the photosensitive material to pass therebetween, oscillatorrectifier means for supplying one of said charging devices with a negative potential and the other with a positive potential, and a filter network with an energy storage capacity in the order of about .072 joule electrically interconnecting said oscillator-rectifier means with each of said charging devices so that when either charging device develops a short circuit, the potential supplied by said oscillator-rectifier means will be interrupted.

2.- Apparatus of the type defined in claim 1 wherein said oscillator-rectifier means includes a metal rectifier electrically interconnected with an electronic oscillator for supplying one of said charging devices with a positive potential, and a high vacuum valve rectifier electrically interconnected with another electronic oscillator for supplying the other charging device with a negative potential.

References Cited in the file of this patent UNITED STATES PATENTS 2,297,691 Carlson Oct. 6, 1942 2,588,699 Carlson Mar. 11, 1952 2,621,309 Faudell Dec. 9, 1952 2,716,826 Huebner Sept. 6, 1955 2,777,745 McNaney Jan. 15, 1957 2,777,957 Walkup Jan. 15, 1957 2,811,465 Greig Oct. 29', 1957 2,864,756 Rothacker Dec. 16, 1958 2,885,556 Gundlach May 5, 1959 2,922,883 Giaimo Jan. 26, 1960 OTHER REFERENCES Perry: Chemical Engineers Handbook, McGraw-Hill (1950), pages 1745-1747.

Young et al.: R.C.A. Review, Dec. 1954,. pages 469- 4-74. 

